Centrifugal compressors are routinely used for medium to large capacity water chillers used for air conditioning or process applications, with a chilled water temperature leaving the chiller to the space to be cooled typically of the order of about 7° C. (45° F.). In order to generate energy savings and benefit from renewable energies, there is a growing demand for heat pumps. In some applications, the “cold source” of such heat pumps can be at a relatively high temperature fluid, for instance, when the heat pump is used to boost the temperature of geothermal water. Due to the great variety of possible applications, the leaving chilled water temperature from the evaporator of heat pumps can vary over a very wide range, typically from 5 to 60° C. (41-140° F.). In the lower side of this temperature range, conditions at the evaporator are similar to those of a standard water chiller; therefore, the design of a heat pump for such applications is very close to that of a standard water chiller. But as the temperatures of the leaving chilled water temperature at the evaporator rises, the leaving chilled water temperature eventually reaches a point where the standard water chiller technology can no longer be used.
Compressors are a key component in HVAC systems, and compressor operating conditions are defined by the evaporating and condensing pressures and temperatures. Some compressors are so-called hermetic and semi-hermetic compressors. These compressor units have the motor sealed inside a common housing with the compressor. The motor operates in an atmosphere of refrigerant, the refrigerant surrounding and cooling the motor. The only major difference between a semi-hermetic compressor and a hermetic compressor is that the housing for a semi-hermetic compressor comprises flanges that can be disassembled to service the compressor or motor. Hermetic compressors are usually of smaller size, like those of household refrigerators or window air conditioning. They are completely canned in a sealed enclosure and cannot be disassembled. Compressors that are neither semi-hermetic nor hermetic are driven by motors that are outside of the refrigerant circuit and which are cooled by non-refrigerant fluid, such as air or water. These compressors are referred to as open compressors. This invention finds particular applicability to semi-hermetic compressors and hermetic compressors, although it may find use in open compressors. The terms semi-hermetic, hermetic, semi-hermetic compressors and hermetic compressors may be used interchangeably herein.
The difference between evaporating and condensing temperatures associated with evaporating and condensing pressures is typically of the order of delta (Δ) 50° C. ((Δ) 90° F.). In the upper range of temperatures for heat pumps, the evaporation temperature can be as high as 60° C. (140° F.) or even higher. Taking into account a normal pinch on the evaporator, the evaporation temperature is typically about (Δ) 2° C. ((Δ)3.6° F.) lower than the leaving water temperature from the evaporator, resulting in a leaving water temperature of about 62° C. (144° F.) when the evaporation temperature is 60° C.
Water chillers and heat pumps using centrifugal compressors normally use synthetic refrigerant fluids derived from hydrocarbons. Because of environmental concerns, several families of synthetic refrigerants have been used, are being used, or are under development, belonging to the families of CFC's, HCFC's, HFC's or HFO's. Most centrifugal chillers in operation today are using HFC-134a. For the higher temperature range of heat pump applications, the tendency is to use lower pressure refrigerant fluids like HFC-245fa. These HFC's are likely to be replaced to a certain extent by future generation hydrofluoro-olefins (HFO's).
In the lubrication circuit of a typical centrifugal compressor, oil is collected from the lower part of the oil sump. It is circulated by an oil pump and pressurized to send it to the bearings and to the other points in the compressor requiring lubrication, for example, the gears for a gear-driven compressor, and also the shaft seal. After providing lubrication, the oil is drained and returned to the oil sump by gravity. The system is complemented by an oil cooler, usually located at the pump discharge before injection of lubricant into the compressor. The oil cooler has the effect of eliminating heat generated by mechanical friction generated in the compressor, for instance in the bearings and in the gears that is absorbed by the lubricant. An oil heater is also installed in the oil sump to keep the oil sufficiently warm when the compressor is not operating, so as to provide a lubricant of suitable viscosity to properly lubricate the compressor on start-up.
In lubricated compressors used in refrigerant circuits, the lubricating oil, a liquid, is in the presence of a gas refrigerant in the oil sump and various parts of the lubrication oil circuit. In centrifugal or reciprocating compressors, the pressure in the oil sump is usually equalized or vented at or close to the suction pressure of the compressor. This function is performed by a gas-equalizing line collecting gas refrigerant from the upper part of the oil sump. The collected gas refrigerant is returned to the low pressure side of the refrigerant circuit, such as the evaporator or compressor suction. The reason for this venting is related to the mutual miscibility between lubricating oils and most of the refrigerants, and to the effect of this miscibility on the oil viscosity. The viscosity of a blend of oil and refrigerant depends not only on the temperature, but also on the dilution of refrigerant in the oil. This dilution depends on the temperature of the refrigerant and oil and the pressure of the refrigerant gas. The general tendency is that the amount of refrigerant in solution in the oil increases as the temperature decreases, while increasing the dilution by the refrigerant tends to reduce the viscosity. Due to this mechanism, lowering the temperature of the refrigerant and oil tends to reduce the oil viscosity; this is opposed to the normal tendency for pure oil, where the viscosity decreases as the temperature increases. Therefore, the refrigerant in solution in the oil and the resulting viscosity are in a complex relationship, depending on the fluid temperature, the refrigerant pressure, and the mutual miscibility of the oil and refrigerant. Besides having the effect of reducing the oil viscosity, the dilution by refrigerant in the oil can have other adverse effects. The main one is oil foaming in some parts of the circuit in case of pressure reduction or temperature increase. This can result in undesirable cavitation of oil pumps, or drastically reduced lubricity, potentially resulting in mechanical failures.
The refrigerant in the lubrication circuit comes from two sources. The first source of refrigerant gas is in the circulating oil itself. The path of the oil within the compressor for lubrication purposes places the oil in contact with refrigerant. Some refrigerant can enter into the oil lubrication circuit in both a gas phase and a liquid phase. As the oil is in the presence of gas refrigerant in many parts of the refrigeration circuit, the oil tends to absorb some refrigerant. Gas refrigerant from locations of higher pressure in the compressor also migrates to the sump, which is at a lower pressure. A typical example is the gas leakage from and around the labyrinth seals. Likewise, in a reciprocating compressor, some of the compressed refrigerant gas will leak through the piston rings and migrate into the sump. In addition, the lubrication process may induce some high agitation of the oil resulting in oil foaming. Examples include lubrication of high speed gears or oil splashing resulting from the crankcase rotation in a reciprocating compressor. It should be noted that the oil return circuit also may introduce a substantial amount of liquid refrigerant into the sump, and not all of the liquid refrigerant entering the sump flashes off immediately. Due to this complex mechanism, some refrigerant must be permanently removed from the compressor oil sump. One purpose of the oil sump is to provide the oil an opportunity to settle and release refrigerant gas bubbles before being re-circulated in the lube oil circuit. Even after this gas separation, some refrigerant remains dissolved in the oil that resides in the sump. The vapor space above the oil in the sump is usually vented directly to the compressor suction, which is at pressure only slightly lower than that of the evaporator. The slightly higher pressure in the sump forces the gas refrigerant that is separated to be reintroduced into the compressor at its suction point as a vapor. In the case of a centrifugal compressor, the total amount of refrigerant that needs to be removed from the sump is typically of the order of 1 to 3% of the total flow of the compressor.
In heat pump applications, the evaporation pressure tends to be substantially higher than in water chillers, which increases the amount of refrigerant absorbed by the oil, tending to decrease the oil viscosity and reduce its lubricity. The oil temperature also should be set to a higher value in order to keep the oil dilution level at an acceptable value, further reducing the oil viscosity. To compensate for this effect, an oil grade with higher viscosity can be used. But even with this compensation for the viscosity, the temperature elevation raises other issues. Among these is a risk of failure of the shaft seals and bearings when the oil temperature is too high. There is no fundamental reason why this issue could not be resolved to a certain extent, but it may require time consuming and expensive validations leading to out-of standard and more expensive solutions. Therefore, what is desired is a system that would compensate for some of the differences between standard chillers and higher temperature heat pump conditions. This would also allow extending the range of application of standard air conditioning compressors beyond chiller applications to heat pump applications.
To keep costs low for heat pumps used in systems such as geothermal systems, and to minimize complications for technicians and other service personnel, it is desired to maintain equipment design and commonality for chillers used as high temperature heat pumps as close as possible to those used for standard water chilling systems. However, systems utilizing a substantially higher evaporation temperature, such as used in heat pump applications, raise a number of questions, especially related to the lubrication system and motor cooling, as well as to the lubrication of the shaft seal in designs employing an open compressor. What is needed is a system that can reduce the amount of refrigerant absorbed by the oil so the lubricity of the oil is not adversely affected.